The invention relates to a high-pressure gas discharge lamp with a cooling arrangement, and to a lighting unit comprising such a lamp.
High-pressure gas discharge lamps (HID [High Intensity Discharge] lamps) and in particular UHP (Ultra High Performance) lamps are preferably used inter alia for projection purposes because of their optical properties.
A light source which is as point-shaped as possible is required for these applications, i.e. the light arc arising between the electrode tips should not have a length which exceeds a value of approximately 0.5 to 2.5 mm. Furthermore, a luminous intensity which is as high as possible is desired, accompanied by as natural a spectral composition of the light as possible.
These properties can be optimally achieved with UHP lamps. In the development of such lamps, however, two essential requirements must be complied with simultaneously.
On the one hand, the highest temperature at the inner surface of the discharge space must not become so high that a devitrification occurs of the lamp bulb, which is usually manufactured from quartz glass. This may be a problem because the lamp is particularly strongly heated in the region above the light arc owing to the strong convection inside the discharge space.
On the other hand, the coldest spot on the inner surface of the discharge space (or burner space) must still have a temperature which is so high that the mercury is not deposited there, but remains in the evaporated state to a sufficient degree. This is to be heeded in particular in the case of lamps with a saturated gas filling.
These two mutually conflicting requirements have the result that the maximum admissible difference between the highest and the lowest temperature (usually of the upper and lower inner surfaces of the discharge space) is comparatively small. Complying with the maximum for this difference is comparatively difficult, and narrow limits are imposed on a power rise of the lamp because it is mainly the region above the discharge space which is heated by the interior convection, and the temperature thereof can be reduced to a limited degree only through a suitable shaping of the lamp bulb.
Finally, these requirements often cause a problem also if the light output of the lamp is to be dimmed, because this leads to a cooling-down and condensation of the gas, and accordingly to a degradation of the spectral properties of the generated light in most cases.
It is accordingly an object of the invention to provide a high-pressure gas discharge lamp of the kind mentioned in the opening paragraph, and in particular a UHP lamp suitable for projection purposes, whose spectral properties are clearly improved over a wider power range.
A further object is to provide a lighting unit with a high-pressure gas discharge lamp as well as a power supply unit by means of which such a lamp can be operated such that its spectral properties are clearly improved over a wider power range.
The object mentioned first is achieved, according to claim 1, by means of a high-pressure gas discharge lamp with a cooling arrangement, which is characterized in that the lamp can be operated at an increased power level such that an increased gas pressure is generated by an increase in the temperature (in general of the coldest spot) in the lamp interior, while the cooling arrangement is positioned and dimensioned such that a devitrification of the lamp bulb and a condensation of the filling gas are substantially prevented at said increased power level.
An essential advantage of this solution is that not only the spectral properties of the light are clearly improved, but also that the lamp operates as a higher operating voltage because of the higher gas pressure, so that a correspondingly higher lamp power is achieved for a given lamp current. On the other hand, a smaller current is required for a given lamp power. The result of this is that the electrodes, which are normally subjected to a particularly strong wear in the case of the electrode distances of approximately 0.5 to 2.5 mm which are of interest for projection applications, now have a substantially longer operational life. All this can be achieved without any change in the geometry of the lamp.
The second object mentioned above is achieved, according to claim 7, by means of a lighting unit comprising a high-pressure gas discharge lamp according to the invention as well as a power supply unit for operating the lamp, characterized in that the power supply unit comprises a first control circuit for supplying the lamp with a power at which an increased gas pressure is generated through an increase in the temperature (in general of the coldest spot) in the lamp interior, said first control circuit comprising an output terminal to which an information signal relating to the level of the lamp voltage is applied and which is arranged so as to be connected to a second control circuit for operating a power source which generates the flow of cooling agent in dependence on the level of the lamp voltage such that both a devitrification of the lamp bulb and a condensation of the filling gas are substantially prevented.
It is an advantage of this solution that the lamp and the cooling arrangement can be operated in a manner such that they are mutually attuned. This relates in particular to the adjusted output power of the lamp and the lamp voltage, because the latter is dependent on the gas pressure in the lamp, so that the luminous output power can be increased by a factor of between approximately 1.5 and 3 as compared with the rated power of the lamp without cooling and without a devitrification of the lamp bulb being observable.
It should be noted here that a halogen metal-vapor lamp is known from JP-6-52836, which lamp comprises an air channel by means of which an air flow is directed to an upper portion of the outer surface of a luminous tube. The object of this air flow is to prolong the operational life of the lamp by means of a temperature distribution which is as homogeneous as possible. Apart from the fact that an improvement in the spectral properties of the light cannot be achieved thereby, there is also a problem here that the temperature of the coldest spot is particularly sensitive to any air flow because the temperature gradient in situ (i.e. at the lower side of the lamp bulb) is substantially narrower than at the upper side. The admissible range for the air flow by which no condensation of mercury is caused is accordingly very narrow, so that high requirements are imposed on the accuracy of the cooling system and narrow tolerances are to be observed. On the other hand, the spectrum of the radiated light and the burning voltage are even impaired and reduced, respectively, owing to the condensation of mercury. It is further proposed to provide a glass plate in horizontal position in the reflector so as to prevent an undesirable cooling-down of the lower side of the lamp. This measure, however, not only involves a considerable expenditure, but it also adversely affects the optical output power of the lamp.
The dependent claims relate to advantageous further embodiments of the invention.
The embodiment of claim 2 is particularly advantageous in the case in which the lamp power is adjustable.
The effectiveness of the cooling is further improved with the embodiments as defined in claims 3 to 6, so that the lamp power can be further increased or the lamp current can be correspondingly reduced, while at the same time the spectral properties of the light are further improved.
The embodiment of claim 8 relates to a complete lighting unit with a power supply unit for the lamp as well as for the cooling arrangement, so that economic advantages arise from the integration.
The embodiment of claim 9 involves an optimization of the cooling in dependence on the output power of the lamp, so that according to claim 10 also dimming of the output power is possible without detracting from the spectral properties of the light. The embodiments of claims 11 and 12 have particular advantages as regards a fast switching-on and restarting of the lamp.